Moisture or frost is formed on the windows of a vehicle when there is a temperature difference between the outside and the inside of the vehicle. Heating glass may be used in order to solve this problem. Heating glass uses a concept of generating heat from a heating line by attaching a heating line sheet to the glass surface or directly forming a heating line on the glass surface and applying electric power to both terminals of the heating line, and thereby increasing a temperature of the glass surface.
Particularly, methods employed for providing heating to vehicle front windows while having excellent optical properties are largely divided into two types.
The first method is forming a transparent conductive thin film on the whole window surface. The method of forming the transparent conductive thin film includes a method of increasing transparency by using a transparent conductive oxide film such as ITO, or by forming a thin metal layer and then using transparent insulation films above and below the metal layer. This method has an advantage in that an optically superior conductive film may be formed, however, there is a disadvantage in that a proper heating value may not be obtained at low voltage due to relatively high resistance.
The second method may use a method of using a metal pattern or wire, and increasing transparency by maximizing a region having no patterns or wires. Typical products using this method include a heating glass produced by inserting a tungsten wire to a PVB film used for bonding vehicle front windows. In this case, the diameter of the used tungsten wire is 18 micrometers or greater, and conductivity capable of securing a sufficient heating value at low voltage may be obtained, however, there is a disadvantage in that the tungsten line is noticeable due to the relatively high thickness of the tungsten line. In order to overcome this problem, a metal pattern may be formed on a PET film through a printing process, or a metal pattern is formed through a photolithography process after attaching a metal layer on a PET film. A heating product capable of heating may be produced by inserting the metal pattern-formed PET film between two PVB films, and then going through a glass bonding process. However, there is a disadvantage in that a PET film is inserted between two PVB films, and therefore, there may be a distortion in the objects seen through vehicle windows due to a refractive index difference between the PET film and the PVB film.